Pioneer SA-610 Service-Handbuch - Seite 9

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7 . C I R C U I T D E S C R I P T I O N S

Equalizer Amplifier

The 54,-610 features a 3-stage direct-coupled

equalizer amplifier for greater reduction of noise

and distortion.

Besides the use of an ultra low-noise transistor

(25C26O2) in the first stage, the adoption of low

impedance input resistance and equalizer circuit

has contributed to an S/N ratio of at least 82dB

(at 2.5mV input, IHF-A).

Power Amplifier

This all stage direct-coupled pure complemen-

taly SEPP circuit features a current mirror load

differential amplifier in the first stage, and incor-

porates the tone control circuit in the NFB loop.

Although the incorporation of the tone control

circuits in the power amplifier stage reduces the

number of elements that the signal has to pass

through, and thereby further reduces noise, dis-

tortion, and cost, the power stage does require a

higher gain and a higher degtee of stability. In the

SA-610, this high gain and high stability

are

achieved by a cunent

mirror

load differential

amplifier in the first stage and by the use of a

constant current circuit for the load of the pre-

driver stage. Furthermore, the tendency for the

pre-driver stage capacitance impedance load to

cause deterioration

in the high end frequency

response is suppressed by inserting a capacitor (C)

between the emitter of the pre-driver stage (Q2)

and the base of the constant current circuit (Q1) as

shown in Fig. 7-1.

At frequencies where the

reactance of this capacitor may be ignored, the

push-pull action of Ql and Q2 serves to counteract

the high end frequency response deterioration.

The power amplifier stage is a complementary

2-stage' Darlington

connection,

resulting in an

output power rating of 45W + 45W (8Q, 20Hz-

2OkHz\, harmonic distortion of less than O.O37o

(20H2

20kHz at rated output),

and output

power bandwidth of 5Hz - 50kHz (0.O3% THD).

Certainly a superb performance for an amplifier

of this class is obtained.

/. Constant

current

c i r c u i t

)to

o,,u.,..r"n"

\Pre-driver

staga

siA-61 cl

Protection Circuit

Besides protecting the speakers if a DC voltage

should happen to appear at the power amplifier

stage outputs, this circuit also mutes the signal

path when the power switch is turned on and off

(See Fig. 7-2).

If for any reason a DC voltage (in excess of

about tbV) should happen to appear in the output

of the power amplifier stage, it is detected imme-

diately by either Q23 or q24. qn

is turned on by

a positive voltage, and Q24 by a negative voltage. In

either case, Q25 is also turned off, thereby opening

the relay contact to disconnect the power amplifier

stage from the speakers. ,

The muting action when the power switch is

turned on is achieved by delaying the rise of the

Q25 base potential by means of the R95/C77 time

constant circuit. When the power switch is turned

on, C77 is charged up via R95, thereby increasing

the voltage across both ends of this capacitor.

When this voltage exceeds the zener voltage of the

D15 zener diode, Q25 is biased in the forward

direction, and is thereby tumed on to close the

relay contact.

When the power switch is turned off., C77

discharges rapidly via D18, resulting in Q25 being

turned off, thereby opening the relay contact.

L C H S P O U T

R CH SP OUT

Fig. 7-2 Frotection circuit

!f'

r l

i F

Fig. 7-1 Pre-driver stage